As to their electrical properties, polymers and polymer mixtures are usually dielectrics. However, in some applications the polymer material preferably has some electrical conductivity. Such applications include antistatic packages, containers for inflammable substances and pipework, electrostatically painted fabricated shapes, and many other applications known per se. In addition, the increasing number of electronic devices has resulted in that, on the one hand, their electronics have to be protected from electromagnetic interference caused by other electronic devices and, on the other hand, the electronic interference caused by them on the outside has to be reduced. In other words, the devices are to be EMI shielded. Electrical conductivity is often achieved by mixing metal or metal-containing particles, coal, graphite or combinations thereof in a substantially non-electrically conductive polymer acting as a matrix material.
The term elastomer refers to a material formed from macromolecules and characterized by extensibility and rapid recovery to the original shape after the tension is released. Electrically conductive elastomer composites are known wherein the matrix material is of a thermoset material, a silicone polymer, for example. Such a matrix material has to be crosslinked to achieve elastic properties, and, generally, to enable the handling of the product. Crosslinking requires much energy and time, and special crosslinking means, making the manufacture of the products slow and the production costs high. Electrically conductive thermoplastic elastomer composites are also known art. These materials can be processed rapidly and inexpensively compared with thermoplastic materials, but the resistivity values are usually higher than with thermoplastic materials.
In order for an elastomer composite to be electrically conductive, the electrically conductive particle mixed therein must be in contact with each other or the distance between them has to be small enough to allow an efficient tunnelling current to pass between them. In addition, chains leading through the matrix material have to be generated from the particles (G. R. Ruschau et al, J. Appl. Phys. 72, (1992) 953-959). The volume fraction of electrically conductive particles has to be large enough for said condition to be fulfilled. However, increasing the volume fraction deteriorates (among other things) the mechanical properties, workability and surface quality of an elastomer composite. Furthermore, the price of the material often increases significantly. Consequently, the volume fraction cannot be increased limitlessly to increase electrical conductivity.
It is known to improve the electrical conductivity of electrically conductive fillers by treating the surface of the particles in a variety of ways. An alternative is direct coating of the particles with an electrically conductive polymer. Nickel particles, for example, have been coated with polypyrrol using a method where sodium dodecyl sulphate (SDS) was first applied to the surface of the nickel particles (Genetti W. B. et al, J. Mater. Sci. 33 (1998), 3085-3093). SDS is a surface active substance and forms a double layer around the nickel particles. The pyrrol was then subjected to polymerization inside the double layer. The treatment significantly improved the specific conductivity of a particle-filled polyethylene.
However, a significant problem is associated with known electrically conductive thermoplastic elastomer composites: when subjected to compressive force, the resistivity of the composite rises greatly. For example, when the compression is about 10%, the resistivity is up to 100-fold or more compared with an uncompressed material. Naturally, this is an essential change in the characteristic of a material. According to a theory presented, the resistivity should decrease by the action of compression (G. R. Ruschau et al, J. Appl. Phys. 72, (1992) 953-959). Often in practical applications, some compression is directed to products made from said material, e.g. seals, and thus the problem is extremely common and harmful.